Material classifiers are used for many different purposes, including the separation or classification of solids according to size and/or particle density. Many different types of material classifiers are known, including mechanical and non-mechanical types.
According to one type of material classifier, solids to be separated are mixed in a suitable liquid such as water, to create a liquid-solid mixture or pulp. The mixture is then introduced into a classifier tank. Larger particles settle to the bottom of the classifier tank while fine particles remain in suspension in the liquid medium (called the overflow). A driven wheel having flights, lifts, drags, blades, scoops, scrappers or other means is used to lift solid material which has settled on the bottom of the tank and discharge it upon a discharge chute, conveyor belt or other means for collecting and transporting the settled material. The liquid is drawn the classifier or exits as an overflow. Material classifiers of this type also provide cleaning of the solid particles.
A known material classifier of this first type, an example of which can be seen in U.S. Pat. No. 1,107,472, issued Aug. 18, 1914, uses V-shaped troughs (“buckets”) or scrapers spaced around the circumference of a cylindrical classifier tank or vessel. The vessel is partially filled with water and slowly rotates. Materials lighter than water will float on the water's surface and be discharged from the vessel via an overflow trough. Heavier materials sink to the bottom of the vessel and are scooped-up by the buckets as they rotate. When the buckets reach a specified height within the vessel, the contents of the buckets are dumped onto a spout which discharges the material from the vessel.
Another known material classifier of this first type, an example of which can be seen in U.S. Pat. No. 2,226,750, issued Dec. 31, 1940, uses a circular wheel with radially spaced blades. Heavier solids scooped-up by the blades are pushed to a discharge lip. Lighter solids are kept in suspension and exit the classifier at an overflow point such as a weir. The classifier blades have a cam mechanism allowing the blades to retract as they move upwards beyond the discharge lip. On the downward rotation the blades are lowered into the water edgewise to minimize the liquid surge caused by the blades entering the water.
Another type of classifier typically used for classifying sand and aggregate cleaning use a screw mechanism for moving the sand/aggregate along the classifier. These designs are commonly referred to as rotary-drum or screw-conveyor type classifiers, an example of which can be seen in U.S. Pat. No. 4,151,074, issued Apr. 24, 1979. Screw classifiers can be complex, prone to wear, and can be expensive and costly to maintain and set up.
Another type of classifier uses an elongate classifier tank or trough. The liquid-solid mixture is introduced at a relatively high flow rate at one end of the classifier tank. A number of discharge pipes/outlets are provided near the bottom of the classifier tank along its length. Larger and heavier particles settle closer to the classifier inlet. Smaller and lighter particles remain suspended longer than heavier/larger particles and travel further from the inlet before settling. The liquid exits the classifier tank using an overflow or other device. By opening the appropriate discharge pipes, solid material having the desired particle size/density can be withdrawn from the classifier. Typically, the withdrawn material is subsequently processed by dewatering apparatus, such as a screw conveyor, to remove the water therefrom.
A common drawback of existing classifier designs is that good classifying ability is typically achieved at the expense of capacity and vice versa. Typically, a material classifier has either good classifying ability but low capacity and a complicated reclaiming system, or high capacity and a relatively simple reclaiming system but poor classifying ability. Also, material classifiers with good classifying ability typically offer a much greater classifying ability than is typically required as most fine grade materials have fewer uses.
A further drawback of most material classifiers is that they are large and not easily portable between job sites. Some material classifiers, such as those at a quarry or aggregate pit, are typically large installations requiring a support structure and therefore cannot be transported. Other types of classifiers, for example screw conveyors and driven wheel apparatus, are capable of being transported. However, these types of material classifiers must typically be loaded onto a truck, for example using a forklift, lift truck or crane, transported to the desired location, and unloaded from the truck. In addition to being a source of downtime, loading and unloading of the classifier requires equipment at both the initial and final destinations to perform the loading/unloading operation. Further, these types of classifiers may require some disassembly for transportation and reassembly on arrival.